Website very slow

I need to optimize my web store. Now the page takes around to 15 seconds to open and I can't find what is causing this. 2 months ago the page took 6 seconds to open and no changes was made on Prestashop core. The web works integrated with a importing tool from a Dropshipping seller. Information: - Web: [login to view URL] - Shared hosting: CPU Cores 6 - RAM Availability 6 GB - SSD Storage - PrestaShop: 1.7.2.4 - PHP: 7.1.20 - Custom template. [login to view URL] I found that when the browser cache is clean, the page takes 4 seconds to open, however if I open the page a second time it takes more then 15 seconds to open. I changed the host because initially, I thought that the issue is with host, however the issue is the same at new host. [login to view URL] Other information that can help you to define the budget and of course, if we have an agreement, I'll give you the credentials to verify. - The web is new, but at first that did not happen. We changed Host thinking that the problem was there but it was not like that. Since May, the slow loading began. We have not changed anything in the structure or template, which is why we think the slowness is strange. - The page does not respond about 8 times a day and returns error 500 Connection Timeout stays like this for minutes, sometimes 1 minute, sometimes about 8 minutes.